Doubt on "bias" for texture access Built-in Func

<<OpenGL Shading Language>> stated that " If the bias parameter is not provided, the implementation automatically selects level-of-detail." My doubt is that when the texture access bifuntion(
texture1D/2D/3D())“automatically” decides which mipmap level should be used, what standard should it to live up to?
1.The size of object which is rendered?
2.Or the proportion between the size of object and the width/height of scene?
3.Or any relationship to the size of window?
Is there any formula for computing which mipmap level is to be used when texture1D/2D/3D(without bias assigned) is executed?
Any help will be appreciated.Thank you!

depends on the Filter NEAREST MIPMAP NEAREST, NEAREST MIPMAP LINEAR, … and is written in the OGL spec under Mipmapping.
pp. 177 in glspec21.20061201.pdf
pp. 209 in glspec30.20080811.pdf

Thank you very much!